Bedding mattresses are available in a wide range of types and configurations offering a variety of support conditions for the user.
A principal object of a bed mattress is to provide optimal support for the user commensurate with their physical and medical requirements. Such optimal support requires the mattress to conform substantially to the shape of the users body when resting on the mattress. Unfortunately, most available mattresses react proportionally to the weight distribution of the users body, compressing most where the body is heaviest and least where the body is lightest. This results in mattress conformation which does not reflect the actual physical shape of the users body, but rather reflects a shape imposed on the mattress by the weight distribution of the body. Accordingly, the users body adopts a shape which results from the weight distribution of body segments which does not reflect the actual body shape when resting on a standard mattress. The users skeleton is then twisted and distorted to fit the shape of the mattress as it has reacted to the users weight distribution.
The areas of greatest distortion are the shoulder and lumbar/hip regions. The shoulders are usually the widest part of the human body but occur at the lightest region of the torso. Accordingly the shoulders, when a user is resting on their side, do not push a mattress down much in accordance with the body shape resulting in a degree of twisting of the body when the user is sleeping on their side. In contrast to the shoulder and upper torso region, the hips and lumbar region of the torso are generally much heavier and this region of the users body will compress that part of a mattress disproportionately. In standard mattresses of uniform stiffness this results in the pelvis region being the lowest supported part of the body. Furthermore, the close proximity of the hip region to the waist region of the user tends to deprive the waist, and important lumbar region, of the user with adequate support as the mattress is highly compressed at the hip region and the adjoining area of the mattress leading into the lumbar region is also compressed where it should actually be providing support.
In order to provide optimal support a bed mattress should be able to react independently to the different regions of the users body and at least able to provide dedicated support for the upper, middle and lower torso regions which all have quite distinct weight distribution and support requirements.
An analysis of these three regions designated Region “C” for upper torso; Region “B” for middle torso; and Region “P” for lower torso, highlights the different requirements needed to provide optimal support.
Given that the weight of the lower torso (pelvis region “P”) Wp is about 130% of the weight of the middle torso (Belly region “B”) Wb; and the weight of the upper torso (chest region “C”) Wc is about 50% of the weight of Wb.
ThenWb=Wp/1.3=0.77 Wp Wc=0.5 Wb=0.39 Wp 
If the mattress deflection at region B is minimal—say 15 mm and the lumbar curve of a users spine is about 60 mm then for a mattress of uniform stiffness or elasticity, deflection at region P and region C should be about 15+60 mm=75 mm. Such a deflection will require a spring stiffness Kb of(75/15)×[(1.0/1.3)×Kp]=385% Kp 
The spring stiffness Kc at region C should be(0.5/1.3)×Kp=39% Kp 
In summary, in order to provide optimal support over the region C, B and P the following general variation in spring stiffness would be desirable.
UpperMiddleLowerTorso KcTorso KbTorso Kp0.4Kp3.8KpKpKc9.5Kc2.5Kc0.1KbKb0.26Kb
Such variation in spring stiffness is not usually available in production mattresses. The high cost of producing a mattress with such degrees of variation in stiffness plus the differing height of the end user necessitating different placement of regions C, B and P has prohibited the manufacture and availability of mattresses with such performance characteristics to date.